Interaction of human substantia nigra neuromelanin with lipids and peptides.
Neuromelanin was isolated from human substantia nigra using different procedures. In the pigment isolated by any of these procedures a peptide component covalently bound to the melanic structure was found, as shown by treatment with reagents known to eliminate noncovalently bound proteins. The amino acid content of such a peptide component was reproducible and corresponded to approximately 15% of the neuromelanin weight. Neuromelanin also showed the ability to absorb specifically lipid molecules, approximately 20% of its weight, and among these lipids cholesterol was identified, constituting approximately 5% of the total lipid mixture. A synthetic melanin, incubated with putamen homogenate, bound tissue peptides with an amino acid content quite close to that of neuromelanin. The same synthetic melanin adsorbed a lower amount of lipids from the putamen homogenate compared with neuromelanin. The sulfur content of neuromelanin was also reproducible even using different isolation procedures. A nonpigmented tissue like corpus callosum was used as a control and extracted by the method used for neuromelanin isolation; a total elimination of tissue components was found, thus demonstrating the capability of the reported procedures to isolate neuromelanin alone. The presence of a peptide component in the neuromelanin structure and the selective affinity for lipid molecules suggest new aspects of the functional role and metabolic pathway of neuromelanin.